This invention is in the field of fluid mechanics, and is more specifically directed to static mixers.
Pipelines of varying length and diameter are commonly called upon to transport flow streams having multiple phases; such flow streams are referred to in the art as multiphase flows. Multiphase flow transportation is commonly used in the oil and gas production industry for various reasons. For example, the production output of a hydrocarbon well generally includes multiple phases, namely oil and gas (and, undesirably, water).
Secondary recovery operations refer to those oil and gas production operations in which matter is injected into a producing hydrocarbon well in order to increase the production rate at which salable oil and gas may be retrieved; secondary recovery operations also may extend the producing life of the well. Substances that are commonly injected into hydrocarbon wells include steam, water, and gas (either natural gas, or a non-hydrocarbon gas such as carbon dioxide). In particular, secondary recovery operations involving the injection of gas may inject gas into the formation itself to displace oil that is otherwise resident within the pores of the formation. In some wells, the alternating injection of water and gas into the well has been found to provide improved secondary recovery performance, as gas trapped in the formation alters reservoir fluid mobilities and results in improved waterflood sweep efficiency. As a result, the alternating injection of water and gas has been found to increase production rates and reduce water handling costs.
It has been found that, in certain wells such as those in the North Slope of Alaska, reduction in the water-gas cycle of alternating water and gas injection improves the efficiency of the carbon dioxide recovery process, adds incremental reserves, and improves lift efficiency of the injected wells. As described in U.S. Pat. No. 5,241,408, issued Jun. 6, 1995, commonly assigned herewith and incorporated hereinto by this reference, the simultaneous injection of gas and water into the formation has been found to be extremely beneficial in secondary recovery operations. This simultaneous injection not only achieves the ultimate reduction of the water-gas cycle of alternating water and gas injection, but also reduces the facilities cost associated with separate water and gas piping networks. As discussed in the above-referenced U.S. Pat. No. 5,241,408, successful simultaneous water and gas injection requires uniform distribution of the gas and liquid mixture throughout the piping network.
Splitting or branching of a main multiphase stream into two or more sub-streams is necessary in the piping network used for applications such as simultaneous water and gas injection, to keep facility cost reasonable. In effecting such splitting, however, it is difficult to maintain the same gas/liquid ratio in the substreams as in the main flow, due to the separation of the phases in the multiphase flow at the branching location. In general, the higher density phase (e.g., liquid) tends to flow in the least deviated pipe branch because of its greater momentum, while the lower density phase (e.g., gas) will tend to flow into the most deviated pipe branch. This problem has been addressed in the art by mixing devices, generally referred to as "mixers", an evenly dispersed, or mixed, multiphase flow stream may be accurately split at a branch in the piping system, thus achieving uniform gas/liquid ratio among the branched substreams.
A conventional class of mixers that are of particular benefit in large piping networks are static mixers, which are placed in-stream in the piping network. The mixers are referred to as static, in that the mixing action results from turbulence in the flow stream caused by the structure of the mixer; no motor drive is necessary, making the cost and operation of the mixers attractively low for large and remote piping networks. As described in the above-incorporated U.S. Pat. No. 5,421,408, a conventional static mixer is constructed as a plurality of axially spaced mixing segments, each of which have several radially projecting, circumferentially spaced, pitched blades. The segments and blades are stationary within the piping conduit, and thus serve to impart a spiral direction of flow in the stream passing through the mixer. The relative direction, pitch, and location of the various segments and blades are selected to obtain the desired turbulence and mixing action of the stream. In the piping network described in the above-incorporated U.S. Pat. No. 5,421,408, these static mixers are placed at the locations of branches, for example at across the ports forming the branch intersections or at junctions of conduits.
It has been found, in connection with the present invention, that static mixers of conventional design, while performing reasonably well at high flow rates, do not function very well at low flow rates, because the only available energy for producing the turbulence is the kinetic energy of the liquid itself. In addition, it has been found, in connection with the present invention, that the performance of static mixers of the conventional helical design noted above is quite dependent upon the mixer dimensions. As such, conventional static mixers cannot be readily scaled to various pipe diameters. Accordingly, while a number of helical static mixers are known in the art and commercially available, it is believed that none perform well at low flow rates, and that none are scaleable over a useful range of pipe diameters.